What is a photon?

Discussion in 'The Science Forum' started by Top_Crab, Dec 11, 2011.

Welcome to the Army Rumour Service, ARRSE

The UK's largest and busiest UNofficial military website.

The heart of the site is the forum area, including:

  1. I guess we've all seen those cardboard cut outs representing an electromagnetic wave, with the electric & magnetic fields at right angles to each other. We're told that a photon is a 'Packet' of electromagnetic energy, the thing I would like to know is...How many of these squiggles of waves go to make up a photon?

    Is it one wavelength? Does it depend on the wavelength? How does a photon of Gamma radiation differ from an infra red one?

    Presumably, a photon of Gamma radiation would be 'Shorter' than an infra red one?

    These electric & magnetic fields, are they thin & sharp as represented on the cardboard models? Or are they more of a general 'Splurge'.

    Any non technical simple to understand answers would be greatly appreciated.
  2. A photon can be described as a quanta of electromagnetic energy. The frequency is proportional to the amount of energy a photon has. Long wavelength=lower energy than short wavelength.
  3. Trans-sane

    Trans-sane LE Book Reviewer

    Its based on energy (usually measured in eV). The higher the frequency of the "light" the higher the energy content of the photos that make it up (as determined by an equation that looks like E=fxPlank Constant). Of course its been over a decade since I worked it out so I may well have mis-remembered something ;).

    Shorter wavelength = higher frequency. This is why X-rays and Gamma rays are described as being ionising radiation. They contain enough energy to excite an electron so much it becomes "free" of the atom it was orbiting leaving behind a positively charged ion.
  4. [​IMG]
    • Like Like x 2
  5. Light is both a wave and a particle. Because we can never know for certain the position or momentum of a particle, because [oversimplification] they're ridiculously small, they get represented by a wave peaking at the likeliest position of the particle.

    Fundamentally you have disconnect yourself from the rational, orderly world of classical physics where everything makes intuitive sense. Quantum mechanics makes no concessions to what WE, as human beings, think is hard reality. There's no real point in saying everything is a wave or everything is a particle because that's absurd to our small brains. The best we can say is that it behaves as both.
  6. A photon is a vibrating ring (or string) of energy which can exhibit the characteristics of either a particle or a wave depending upon its mood. The most interesting thing about a photon is it's ability to transport energy from it's source to an object which absorbs the said wavelength and becomes exited by it, i.e. it gets warmed, pies mostly, or pasties, I like pies myself!
  7. I don't think that is quite correct, I think we can measure either the velocity (assuming that is what you meant by momentum) or it's position, not both.
    • Like Like x 1
  8. "Direct measurement of the quantum wavefunction", Lundeen et al, Nature. June 11.

    Theoretically, the wavefunction extends to infinity although this experiment seems to pick them up at about 20mm for visible light.

    "Sharper" rather than "shorter".

    Definitely 'splurge'.

    It is momentum and position - and you are not quite correct either - there is a limit to the accuracy with which we can measure the combination. That does mean that there is a limit to which we can measure either as well. Theoretically, it is down at the Plank length and up at the Plank energy but there is still a fundamental limit.
  9. I'm still confused here, how do you divide up photons? A radio wave may be many meters long, a Gamma ray very much shorter.

    Is a photon one squiggle, many squiggles or what?

    Imagine a single photon travelling through space, what is its length? It sure as shit must have one!

    I suppose a single wavelength should be able to travel through space, would that be the definition of a photon?
  10. Young man, as a very old person - almost old enough to remember glass plates, the definition of a pho ton was as follows :
    1 photo @ 4 x 7 = 0.0017Gm/Iinch
    1 photo @ 5 x 9 = 0,0025Gm/Inch

    Thus the pho ton is a variable which depends upon which size photo you require is as follows:

    a pho ton is 2240lb/sq in divided by the above measurements.

    But trust me. Digital is a lot lighter
  11. Many squiggles. Getting rapidly smaller - more rapidly (even measured by wavelength) the higher the frequency.

    Quite long, actually - look at the twin slits experiment - even at .5mm apart, the slits are going to be very far apart in wavelength terms - roughly a 1000 wavelengths. Yet you can still get interference from a single photon. As the linked paper says, you can detect the fringes of the photon wave function out to about an inch. Just to deliberately mix my units. The maths suggests that it is infinitely long - but then maths is a wee bit less useful than some people think.
  12. How sure are you? When it comes to a photon it's velocity is about light speed but it's momentum is 0.
  13. Please tell me that you don't actually care?
  14. Your lack of curiosity is noted.
  15. A photon does not have momentum = 0(*); its momentum, p, is related to its energy by the expression E = p.c (where c = sol)

    (*)... unless it has zero energy ... and I'm not sure it is meaningful to talk about a photon with no energy.